# What is Kinematics?

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Kinematics is the branch of mechanics in which we study about objects in motion without considering the cause of motion. So basically, in kinematics we study how a body moves without knowing why it moves.

All particles of a rigid body in translation motion move in identical fashion hence any of the particles of a rigid body in translation motion can be used to represent translation motion of the body. This is why, while analyzing its translation motion, a rigid body is considered a particle and kinematics of translation motion as particle kinematics.

Particle kinematics deals with nature of motion i.e. how fast and on what path an object moves and relates the position, velocity, acceleration, and time without any reference to mass, force and energy. In other words, it is study of geometry of motion.

Types of Motion of a Body

Mainly the motion of a body can be of following three types:

(1) Rectilinear or translatory motion.

Rectilinear motion is that motion in which a particle or point mass body is moving along a straight line.

Translation motion is that motion in which a body, which is not a point mass body is moving such that all its constituent particles move simultaneously along parallel straight lines and shift through equal distance in a given interval of time. e.g.,

A body slipping along the inclined plane has translatory motion.
Rectilinear or translatory motion can be uniform or non-uniform.

(2) Circular or Rotatory motion

A circular motion is that motion in which a particle or a point mass body is moving on a circle.

A rotatory motion is that motion in which a body, which is not a point mass body, is moving such that all its constituent particles move simultaneously along concentric circles, whose
centres lie on a line, called axis of rotation and shift through equal angle in a given time.

Circular or rotatory motion can be two dimensional or three dimensional motion and can be uniform
or non-uniform motion. If the circular or rotatory motion is uniform, it is periodic also.

(3) Oscillatory or Vibratory motion

Oscillatory motion is that motion in which a body moves to and fro or back and forth repeatedly about a fixed point (called mean position) in a definite interval of time.

In such a motion the body is confined within well defined limits (called extreme positions) on either side of mean position. It means a periodic and bounded motion of a body about a fixed point is called an oscillatory motion, e.g., the motion of the pendulum of wall clock is oscillatory motion.

If in the oscillatory motion, the amplitude is very small, i.e., microscopic, the motion of body is said to be a vibratory motion. In the oscillatory or vibratory motion, if the restoring force (or torque) is linear, the motion of the body is said to be simple harmonic motion.

What is rest?

An object is said to be at rest if it does not change its position with time, with respect to its surroundings.

What is motion?

An object is said to be in motion if it changes it position with time, with respect to its surroundings.

Are rest and motion relative?

Yes, rest and motion are relative terms. It means an object in one situation can be at rest but in another situation the same object can be in motion. For example, a person sitting in a moving bus is at rest with respect to his fellow passengers but is in motion with respect to objects outside the bus. A person sitting in his house is at rest with respect to earth but is in motion with respect to other planets.

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